Discard Routes and Avoiding Routing Loops

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Discard Routes and Avoiding Routing Loops

• CCNA/CCNP
• Rick Graziani
• Cabrillo College

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Cisco IP Routing by Alex Zinin Addison-Wesley Pub
Co ISBN 0201604736

• This book is highly recommended for instructors
  and CCNP students.
• Special thanks to the author, Alex Zinin, for his
  help.

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Topics

• Situation Normal
• Link Down
• Solution 1 no ip classless
• Solution 2 discard route
• What about packets destined for 192.168.1.0/24?
• Final Notes

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Scenario
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Scenario

• Customer Network is running a dynamic routing
  protocol.
• All subnets within the 172.16.0.0 network are
  contained in this Customer Network. (There are
  no discontiguous 172.16.0.0 subnets via ISP.)
• Remote Office has 172.16.4.0/24 and
  192.168.1.0/24 networks.
• All default traffic is sent to ISP, via 0.0.0.0/0
  default route on RTA that is propagated to RTB
  and RTC.
• ISP has static routes pointing to RTA for
  172.16.0.0/16 and 192.168.1.0/24 networks.

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Situation Normal

• Situation Normal
• As long as all the networks are up, everything
  should work fine.
• Actually, we will see later that even when all of
  our links are up, there could be a problem.

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Link Down
X

• Link down
• What would happen if our link between RTB and RTC
  failed?
• Obviously, the Remote Office networks would be
  cut-off from the Central Office.
• After the routing tables are updated, where would
  RTA or RTB send packets with the destination IP
  address 172.16.4.10?

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Link Down
X

• After the routing tables are updated, where would
  RTA or RTB send packets with the destination IP
  address 172.16.4.10?
• If the router is running ip classless, it will
  forward the packet using the default route. (For
  information on the ip classless command, see the
  presentation on Routing Table Structure.)
• These packets will eventually be sent from RTA to
  ISP.
• Now what will the ISP router do with these
  packets for 172.16.4.1?

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Link Down
X

• Now what will the ISP router do with these
  packets for 172.16.4.10?
• Since it has a route for this network pointing
  back to RTA, it will send it back to RTA.
• This is known as a blackhole in the network.
• Now we have a routing loop!
• Theses packets will eventually be dropped when
  the TTL (Time-to-live) field, in the IP headers,
  is decremented to 0.
• Is there a solution?

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Link down Solution 1 no ip classless

• Link down Solution 1 no ip classless
• One solution could be to change from classless
  routing behavior to classful routing behavior
  using the command no ip classless on RTA and
  RTB.
• The affect of this modification is
• The router would search its routing table for a
  best-match for 172.16.4.10.
• The router would find the parent network,
  172.16.0.0, and search the known subnets, child
  routes, 172.16.1.0/24 and 172.16.2.0/24, but
  would not find the route 172.16.4.0/24.
• The affect of the no ip classless command makes
  the router drop any packets within the 172.16.0.0
  network where there is no known subnet, but there
  is a parent network. (In this case 172.16.4.0/24
  is not known.)
• With the no ip classless command, the router
  does not use any supernet or default route when
  the there is at least one known subnet.
• The packets for 172.16.4.10 would be dropped by
  RTA and RTB.

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Link down Solution 1 no ip classless

• Although this is a remedy, this method cannot
  always be used.
• We will see that this does not solve the problem
  for packets destined for 192.168.1.0/24 when this
  link is down.
• In some situations you may need to have classless
  routing enabled (ip classless)
• May have discontiguous subnets and relying on
  default routing to reach them.
• May be using route summarization and relying on
  supernet routes to reach those specific
  destinations.
• In any case, modifying the route look-up process
  with no ip classless is not an ideal solution
  when you are only trying to solve a specific
  problem, as this might have other affects on the
  routing behavior in your network that you did not
  foresee.

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Link down Solution 2 Discard Route
X

• Link down Solution 2 Discard Route
• A more elegant and scalable solution is to use a
  discard route.
• A discard route is a route that sends packets to
  null0, the bit-bucket, when they do not have a
  route in the routing table and you do not want
  them to be sent using the default route.
• Discard Route on RTA ip route 172.16.0.0
  255.255.0.0 null0
• This would cause RTA to drop all packets for
  subnets in the 172.16.0.0 network, that do not
  have a specific route in the routing table.
• Using our failed route example and still using
  classless routing (ip classless), any 172.16.0.0
  packets not matching 172.16.1.0/24 or
  172.16.2.0/24, would be routed to null0, using
  the discard route.

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Link down Solution 2 Discard Route
X

• Discard Route on RTA ip route 172.16.0.0
  255.255.0.0 null0
• Remember when we said earlier, that even when all
  of our links are up, there could still be a
  problem.
• Well, this also fixes any blackholing of traffic
  we might have for packets that are sent to
  172.16.0.0/16 subnets that do not exist, for
  example, any packets incorrectly sent to
  172.30.5.0/24 subnet. (This is even the case when
  all the links are up.)
• RTAs discard route will drop these packets as
  well!

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Discard Routes
X

• What about packets destined for 192.168.1.0/24?
• In this case, the no ip classless command would
  not help, as this is not a subnet of a parent
  network in the routing tables of RTA or RTB.
• This route would be removed from the routing
  tables of RTA and RTB and all packets would be
  sent to ISP.
• Again, the ISP would send those packets back to
  RTA, causing another blackhole.

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Discard Routes
X

• What about packets destined for 192.168.1.0/24?
• In many cases your networks can be summarized
  within a single supernet.
• In these cases, your discard route should cover
  the entire range of your network, without
  including those routes outside your network.
• The discard route should cover all destinations
  in the central and remote offices, but on the
  other hand should be as specific as possible to
  prevent blackholing of traffic going to other
  subnets of the same summary. Alex Zinin, Cisco
  IP Routing

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Discard Routes
X

• What about packets destined for 192.168.1.0/24?
  (Continued)
• However, in this case the there are networks
  between 172.16.0.0/16 and 192.168.1.0/24, so a
  single discard route will not work.
• What about adding a second discard route on RTA?
• 2nd Discard Route ip route 192.168.1.0
  255.255.255.0 null0
• Unfortunately, this would cause RTA to drop all
  packets for 192.168.1.0/24 whether the link was
  up or not, because this static route would
  replace any dynamic route for this network in the
  routing table. (Lower administrative distance)

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Discard Routes
X

• What about packets destined for 192.168.1.0/24?
  (Continued)
• What about modifying the administrative distance?
• 2nd Discard Route ip route 192.168.1.0
  255.255.255.0 null0 200
• Now, this route would only enter the routing
  table for RTA, when the dynamic route to
  192.168.1.0/24 is deleted.

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Discard Routes

• So, to protect our network from blackholing
  traffic for routes that are in our network but no
  longer reachable (172.16.4.0/24 and
  192.168.1.0/24 examples), or for routes (subnets)
  that our ISP is routing back to us (172.16.5.0/24
  example), discard routes can be used.
• This also is independent of the ip classless or
  no ip classless configuration.
• RTA discard routes
• ip route 172.16.0.0 255.255.0.0 null0
• ip route 192.168.1.0 255.255.255.0 null0 200

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Discard Routes

• Final Notes
• Discard routes can be especially useful for
  networks that use static routes instead of
  dynamic routing, in order to prevent routing
  loops.
• Configuring discard routes is the only way to
  prevent routing loops when several major networks
  are summarized into a supernet. Alex Zinin,
  Cisco IP Routing
• For examples of these and other scenarios, read
  Cisco IP Routing, by Alex Zinin.
• For more information on the affect of the ip
  classless and no ip classless commands, read
  my presentation on The Routing Table -Structure,
  Lookups, and the ip classless command.

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Cisco IP Routing by Alex Zinin Addison-Wesley Pub
Co ISBN 0201604736

• Looking for more?
• This is the book to get!

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Questions?

• Instructors If you have any questions or
  comments, or if you find any mistakes in this
  presentation, please contact me
• Rick Graziani
• graziani_at_cabrillo.cc.ca.us
• www.cabrillo.cc.ca.us/rgraziani